Method and apparatus for employing a light shield to modulate pixel color responsivity

ABSTRACT

A method and apparatus for employing a light shield to modulate pixel color responsivity. The improved pixel includes a substrate having a photodiode with a light receiving area. A color filter array material of a first color is disposed above the substrate. The pixel has a first relative responsivity. A light shield is disposed above the substrate to modulate the pixel color responsivity. The light shield forms an aperture whose area is substantially equal to the light receiving area adjusted by a reduction factor. The reduction factor is the result of an arithmetic operation between the first relative responsivity and a second relative responsivity, associated with a second pixel of a second color.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to color sensors andspecifically to a method and apparatus for employing a light shield tomodulate a pixel's color responsivity.

[0003] 2. Description of the Related Art

[0004] Imaging devices can typically employ a sensor (not shown) todetect light and, responsive thereto, generate electrical signalsrepresenting the light. A sensor typically includes a light sensingelement (e.g., a photodiode), and associated circuitry for selectivelyreading out the electric signal provided by the light sensing circuit.The light sensing circuit operates by the well-known photoelectriceffect that transforms light photons into electrons that constitute anelectrical signal.

[0005] Color imaging devices employ color filter arrays (herein referredto as CFAs) to generate color output. CFAs include a plurality of CFAelements that typically include red, green and blue elements.

[0006]FIG. 1 illustrates a perspective view of a conventional imagingdevice 2 that includes an IR blocking filter 4, a lens assembly 5, andan imager and package 6. The imager and package 6 includes a pixel array7 having a substrate with an active area 8 and a color filter array 9disposed thereon.

[0007] A CFA element 9 is overlaid on the substrate 8 and covers thelight sensing circuit. The combination of the sensor with thecorresponding CFA element is often referred to as a pixel. For example,if a red CFA element is overlaid over a light sensing circuit, thatpixel is referred to as a red pixel. Similarly, if a green CFA elementis overlaid on a light sensing circuit, that pixel is referred to as agreen pixel.

[0008] There are two primary types of imagers. First, there are thoseimagers employing CCD (charge coupled device) technology. Second, thereare imagers that are made using complementary metal oxide semiconductor(CMOS) processes.

[0009] One common problem associated with the use of color filter arrayson CCD and CMOS imagers is that pixel/sensor responsivity varies withthe specific type of color.

[0010] Generally, the responsivity of a pixel of a first color isdifferent than the responsivity for a pixel of a second color. Forexample, in a system employing a red color pixel, a green color pixeland a blue color pixel, assuming a uniform integration time (that is,the time of exposure to light being equal), and a typical scene beingimaged, the signal to noise (S/N) ratio of the pixels will not be equaldue to differing responsivity between the pixels. Blue pixels typicallyhave the least responsivity; consequently, the signal to noise ratio ofthe blue pixels is less than the signal to noise ratio of the red andgreen pixels.

[0011] Moreover, in capturing an image with equal amounts of red, greenand blue light, the pixels having the greatest sensitivity (typicallythe red and green pixels) saturate first. Specifically, the storagecapacitors associated with the red and green pixels reach a maximumcapacity of stored photoelectrons (i.e., saturate) before the bluepixels.

[0012] Once a pixel saturates, the exposure to light is stopped (byclosing a mechanical shutter, for example) to avoid blooming and othersaturation artifacts. Blooming is simply a false electrical signalrepresentation of light intensity at a neighboring pixel because ofcharge leakage from the saturated pixel.

[0013] However, stopping the exposure, although preventing blooming andother saturation artifacts, compromises the signal to noise ratio of thepixels with the lowest sensitivity to light (typically the blue pixels).The consequence of stopping the exposure when the red and green picturesare saturated, is that the pixels with the lowest sensitivity (typicallythe blue pixels) suffer in signal to noise ratio.

[0014] Prior art sensors do not compensate for color responsivityvariation among the different color pixels. Accordingly, when anexposure is made, the exposure time is adjusted to avoid saturation inthe most sensitive pixels. Thus, as a result, blooming is avoided inneighboring pixels. The result of this adjustment in exposure time is adegraded signal to noise (S/N) ratio in the least sensitive pixels(typically the blue pixels).

[0015] A conventional approach is to increase the signal to noise ratioof the blue pixels by increasing the integration time (i.e., theexposure time). However, as one increases the exposure time, althoughthe signal to noise ratio of the blue pixels is increased, the red andgreen pixels saturate and are subject to undesirable saturationartifacts (these undesirable artifacts are commonly referred to in theart as blooming). To counteract the saturation artifacts, the prior artemployed anti-blooming mechanisms in the pixels. However, thesemechanisms increase the cost and complexity of the color pixels.Moreover, these anti-blooming mechanisms are ineffective to eliminatethe blooming effect while still obtaining a desired increase in thesignal to noise ratio of the blue pixels.

[0016] Accordingly, there remains an unmet need in the industry for amethod and apparatus that modifies the responsivity of a color pixel toovercome the disadvantage discussed above.

SUMMARY OF THE INVENTION

[0017] A method and apparatus for employing a light shield to modulatepixel color responsivity. The improved pixel includes a substrate havinga photodiode with a light receiving area. A color filter array materialof a first color is disposed above the substrate. The pixel has a firstrelative responsivity. A light shield is disposed above the substrate tomodulate the pixel color responsivity. The light shield forms anaperture whose area is substantially equal to the light receiving areaadjusted by a reduction factor. The reduction factor is the result of anarithmetic operation between the first relative responsivity and asecond relative responsivity, associated with a second pixel of a secondcolor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The objects, features and advantages of the method and apparatusfor the present invention will be apparent from the followingdescription in which:

[0019]FIG. 1 illustrates an exploded perspective view of a conventionalimaging device.

[0020]FIG. 2 illustrates an exploded perspective view of a pixel cellconfigured in accordance to the teachings of one embodiment of thepresent invention.

[0021]FIG. 3 illustrates an exploded perspective view of the pixel cellconfigured in accordance to the teachings of an alternative embodimentof the present invention.

[0022]FIG. 4 is a flowchart illustrating a method of determining thearea of openings in a metal mask that is used to pattern a metal layerabove each color pixel cell in accordance with one embodiment of thepresent invention.

[0023]FIG. 5 is a flowchart illustrating a method of determiningrelative responsivity of the color pixels in accordance with oneembodiment of the present invention.

[0024]FIG. 6 illustrates a top view of the improved pixel cell before ametal shield layer is deposited thereon.

[0025]FIG. 7 illustrates a top view of the improved pixel cell of thepresent invention after a metal shield layer is deposited thereon.

[0026] FIGS. 8A-8D illustrate a cross-sectional view through A-A of theimproved pixel cell of the present invention, where the first metallayer is employed as a light shield.

[0027]FIG. 8A illustrates a cross-sectional of the improved pixel cellof the present invention after fabrication of active devices byconventional processing methods.

[0028]FIG. 8B illustrates a cross-sectional of the improved pixel cellof the present invention after a first dielectric layer is deposited onthe substrate and via lithography and etch have been performed.

[0029]FIG. 8C illustrates a cross-sectional view of an improved pixelcell of the present invention after deposition of a metal in the viasand a metal polish.

[0030]FIG. 8D illustrates a cross-sectional view of the improved pixelcell of the present invention where a first metal layer is used as ametal shield to modify the color responsivity of the pixel.

[0031] FIGS. 9A-9L illustrate a cross-sectional view through A-A of theimproved pixel cell of the present invention, where the fourth metallayer is employed as a light shield.

[0032] FIGS. 9A-9C correspond to FIGS. 8A-8C and the structures showntherein are made with the same processing steps, described in FIGS.8A-8C.

[0033]FIG. 9D illustrates a cross-sectional view of the improved pixelcell of the present invention after a metal one layer has beendeposited, and the metal one layer lithography and etch processing stepshave been performed.

[0034]FIG. 9E illustrates a cross-sectional view of the improved pixelcell of the present invention after 1) a second dieletric layer (IDL1)has been deposited and polished; 2) a via one lithography and etchprocessing steps have been performed; and 3) a CVD metal deposition andpolish have been performed.

[0035]FIG. 9F illustrates a cross-sectional view of the improved pixelcell of the present invention after a metal two layer has beendeposited, and metal two lithography and etch have been accomplished.

[0036]FIG. 9G illustrates a cross-sectional view of the improved pixelcell of the present invention after a third dielectric layer has beendeposited and polished; via two lithography and etch steps have beenperformed (although not shown in this cross-sectional view); and a metaldeposition and polish have been performed (although not shown in thiscross-sectional view).

[0037]FIG. 9H illustrates a cross-sectional view of the improved pixelcell of the present invention after a metal three layer has beendeposited, and the metal three lithography and etch processing stepshave been performed.

[0038]FIG. 9I illustrates a cross-sectional view of the improved pixelcell of the present invention after a fourth dielectric layer has beendeposited and polished; a via three lithography and etch processingsteps have been performed (although not shown in this cross-sectionalview); and a metal deposition and polish have been performed (althoughnot shown in this cross-sectional view).

[0039]FIG. 9J illustrates a cross-sectional view of the improved pixelcell of the present invention where the fourth metal layer is employedas a light shield to modify the color responsivity of the pixel.

[0040]FIG. 9K is a cross-sectional view of the improved pixel cell ofthe present invention after a dielectric is deposited on the fourthmetal layer.

[0041]FIG. 9L illustrates a cross-sectional view of the improved pixelcell of the present invention after color filter array material has beenspun onto the dielectric material, and the CFA lithography processingsteps of exposure, development, and bake have been performed.

[0042] FIGS. 10A-10C illustrate a cross-sectional view through B-B ofthe improved pixel cell of the present invention where the first metallayer is employed as a light shield.

[0043] FIGS. 11A-11K illustrate the cross-sectional views through B-B ofthe improved pixel cell employing the metal four layer as a lightshield.

[0044]FIG. 12 illustrates a perspective view of an image capture systemin which the improved sensor of the present invention can beimplemented.

[0045]FIG. 13 illustrates a relative response versus wavelength graph.

DETAILED DESCRIPTION OF THE INVENTION

[0046] Referring to the figures, exemplary embodiments of the inventionwill now be described. The exemplary embodiments are provided toillustrate aspects of the invention and should not be construed aslimiting the scope of the invention. The exemplary embodiments areprimarily described with reference to block diagrams or flowcharts. Asto the flowcharts, each block within the flowcharts represents both amethod step and an apparatus element for performing the method step.Depending upon the implementation, the corresponding apparatus elementmay be configured in hardware, software, firmware or combinationsthereof.

[0047] A method and apparatus to compensate the color pixels for colorresponsivity variation so that the signal to noise ratio for thedifferent color pixels are approximately equal while preventingsaturation of pixels of any one color are disclosed.

[0048] The method and apparatus for employing a light shield to modulatecolor pixel responsivity has numerous applications in the imaging field.The present invention can be advantageously employed to increase thesignal to noise ratio of the less sensitive color pixels whilepreventing the saturation of the more sensitive color pixels. Since themore sensitive color pixels do not saturate, blooming and bloomingartifacts are minimized.

[0049] Although the presently preferred embodiment employs a metal layeras the light shield, any opaque material that substantially blocks lightcan be used as a light shield to modulate the color pixel responsivity.Moreover, although in the presently preferred embodiment, the aperturehas a generally rectangular shape, apertures having other geometricshapes including, but not limited to, circular pattern, square pattern,can be employed. Furthermore, the aperture can be symmetric ornon-symmetric, a single aperture or multiple apertures. The importantconsideration is that a portion of the light receiving area, asdetermined by the color responsivity of the pixel and the colorresponsivity of a second color pixel, is achieved.

[0050] Moreover, although the presently preferred embodiment of thepresent invention describes a three color image capture system and aspecific RGB CFA pattern, the teachings of the present invention can beimplemented in image capture systems having fewer or more than threecolors and other CFA patterns. For example, a four color scheme thatemploys cyan, white, green and yellow can be used. For furtherinformation regarding the cyan, white, green and yellow color scheme,please refer to the article entitled “MOS Solid-State Imager”, by AkiyaIzumi and Kohichi Mayama, Hitachi Review, Vol. 32, No. 3, pp. 125-128(1983). Similarly, the present invention can be applied to image capturesystems having different CFA patterns, such as a cyan, magenta, yellowCFA pattern,

[0051] The term pixel cell, as used herein, refers to a light sensingcircuit and a color filter array (CFA) material overlaid on top of alight sensor. For example, a red pixel refers to a light sensing circuitwith a red CFA material overlaid on the light sensor. Similarly, a bluepixel refers to a blue CFA material overlaid on the light sensor. Itwill be known to those of ordinary skill in the art that a light sensorcan include a photodiode and associated circuitry (e.g., transistors) toread out the output of the light sensor.

[0052]FIG. 2 illustrates an exploded perspective view of an improvedpixel cell 200 configured in accordance to the teachings of oneembodiment of the present invention. The improved pixel cell 200includes a sensor substrate 204 having active devices and a lightsensor. The light sensor receives incident light and converts theincident light into an electrical signal representative of said light.The light sensor can be a photoconductor, a photodetector, aphototransistor or photodiode. For example, a metal semiconductorphotodiode such as a PIN structure or a heterojunction photodiode (e.g.,GaAs photodiode) can be employed as the light sensor. Similarly, aSchottky diode structure can also be employed as the sensor.Alternatively, a photo-gate structure can also be used as a lightsensor.

[0053] The sensor substrate includes a photodiode region 208 that is aportion of the pixel cell area 206. The photodiode region 208 has apredetermined light receiving area which can be the entire photodioderegion 208 or a portion thereof. The light receiving area corresponds tothe area of an opening 214 in a metal layer, which is describedhereinafter.

[0054] The improved cell 200 includes at least one metal layer 210 thatforms an opening 214. The area of this opening 214 is important and canbe selectively varied for each different color pixel. Determining thearea of the opening 214 is described in greater detail hereinafter withreference to FIG. 4. Although the incident illumination 220 is incidenton the entire pixel cell area 206, because of the conductive metal layer210, the incident illumination 220 only affects the illuminated area224. It should be noted that the illuminated area 224 for this pixelcell is a fraction of the entire photodiode region 208. In thisembodiment, the metal one (I) layer is employed as a light shield toaffect the color responsivity of the pixel cell.

[0055]FIG. 3 illustrates an exploded perspective view of an improvedpixel cell 300 configured in accordance to the teachings of analternative embodiment of the present invention. The improved pixel cell300 includes a sensor substrate 304 having active devices and aphotodiode. The sensor substrate 304 includes a photodiode region 308that is a portion of the pixel cell area 306. A conductive metal layer310 forms an opening 314 and acts as a light shield. In this embodiment,the light shield is the metal two (II) layer. A metal one (I) layer 330,a metal three (III) layer 334, and a metal four (IV) layer (not shown)are employed in this process for local interconnect. It should be notedthat any one of the metal layers (metal I, metal II, metal III, metalIV) can be employed as the light shield.

[0056] It is important that the metal layers that are not employed asthe light shield not intrude into the photodiode region not covered bythe light shield. It is preferable that metal layers above the lightshield metal layer be kept as far removed as practical from the opening314, to minimize any unintended losses due to shadowing or diffractiveeffects. The metal layers below the light shield layer are restrictedfrom intruding into the photodiode region not covered by the lightshield. It is not necessary that uniform openings be simultaneouslyfabricated in all of the metal layers.

[0057]FIG. 4 is a flowchart illustrating a method of determining thearea of openings in a metal mask for each color pixel cell in accordancewith one embodiment of the present invention. In processing step 400,one of the metal layers (metal I, metal II, metal m or metal IV) isselected to control the pixel cell responsivity. In processing step 402,a determination is made of the relative responsivity of the colorpixels. In the example where there are three color pixels (e.g., redpixels, green pixels and blue pixels), a determination is made of therelative responsivities of the R, G, B pixels (i.e., S_(R), S_(G), S_(B)are determined). In decision block 404, a determination is made as towhich of the color pixels is least sensitive to light.

[0058] If it is determined that the red pixels are least sensitive tolight, processing proceeds to step 410. In processing step 410, themetal mask employed to form the metal above the red pixels is configuredto have an opening with an area substantially equal to the predeterminedlight receiving area for all red pixels. In processing step 412, themetal mask employed to form the metal above the green pixels isconfigured to have an opening having an area substantially equal to thepredetermined light receiving area adjusted by a reduction factor. Inthis embodiment, the reduction factor is the result of an arithmeticoperation between S_(R) and S_(G), and the light receiving area ismultiplied by the reduction factor. The reduction factor can besubstantially equal to S_(R)/S_(G). In processing step 414, the metalmask employed to pattern the metal above the blue pixels is configuredto have an opening having an area substantially equal to thepredetermined light receiving area adjusted by a reduction factor. Inthis embodiment, the reduction factor is the result of an arithmeticoperation between S_(R) and S_(B), and the light receiving area ismultiplied by the reduction factor. The reduction factor can besubstantially equal to S_(R)/S_(B). In processing step 418, the sensordevice is fabricated by employing conventional processing techniques.

[0059] If the green pixel is least sensitive, in processing step 420,the metal mask employed to form the metal above green pixels isconfigured to have an opening having an area substantially equal to thepredetermined light receiving area. In processing step 422, the metalmask employed to form the metal above blue pixels is configured to havean opening having an area substantially equal to the predetermined lightreceiving area adjusted by a reduction factor. In this embodiment, thereduction factor is the result of an arithmetic operation between S_(G)and S_(B), and the light receiving area is multiplied by the reductionfactor. The reduction factor can be substantially equal to S_(G)/S_(B).In processing step 424, the metal mask employed to pattern the metalabove red pixels is configured to have an opening having an areasubstantially equal to the predetermined light receiving area adjustedby a reduction factor. In this embodiment, the reduction factor is theresult of an arithmetic operation between S_(G) and S_(R), and the lightreceiving area is multiplied by the reduction factor. The reductionfactor can be substantially equal to S_(G)/S_(R). Processing thenproceeds to processing step 414.

[0060] If the blue pixels are the least sensitive, in processing step430, the metal mask employed to pattern the metal above blue pixels isconfigured to have an opening having an area substantially equal to thepredetermined light receiving area. In processing step 432, the metalmask employed to pattern the metal above the red pixels is configured tohave an opening having an area substantially equal to the predeterminedlight receiving area adjusted by a reduction factor. In this embodiment,the reduction factor is the result of an arithmetic operation betweenS_(B) and S_(R), and the light receiving area is multiplied by thereduction factor. The reduction factor can be substantially equal toS_(B)/S_(R). In processing step 434, the metal mask employed to patternthe metal above green pixels includes an opening having an areasubstantially equal to the predetermined light receiving area adjustedby a reduction factor. In this embodiment, the reduction factor is theresult of an arithmetic operation between S_(B) and S_(G), and the lightreceiving area is multiplied by the reduction factor. The reductionfactor can be substantially equal to S_(B)/S_(G).

[0061]FIG. 5 is a flowchart illustrating the method steps fordetermining pixel responsivity in accordance with the teachings of thepresent invention. In processing step 500, the input photodioderesponsivity (Resp(λ)) is determined. In processing step 502, inputcolor filter array transmittance for each color T_(R)(λ), T_(G)(λ),T_(B)(λ) is determined. In processing step 504, the input IR blockingfilter characteristics (IR(λ)) are determined. In processing step 506, anet spectral response is computed. The net response is simply theresponsivity multiplied by the input color filter array transmittancemultiplied by the input IR blocking filter characteristic. In processingstep 508, the input light source spectral characteristics aredetermined. The input light source spectral characteristics areconventionally CIE D65 or sunlight. In processing step 510, the netresponse, calculated in processing step 506, is convolved with the inputlight source characteristics, determined in processing step 508, toobtain a relative responsivity for each color (i.e., S_(R), S_(G) andS_(B) are determined).

[0062]FIG. 6 illustrates a top view of the improved pixel cell before ametal shield layer is deposited thereon.

[0063]FIG. 7 illustrates a top view of the improved pixel cell of thepresent invention after a metal shield layer is deposited thereon.

[0064] FIGS. 8A-8C illustrate a cross-sectional view of the improvedpixel cell of the present invention through A-A of FIG. 6, and FIG. 8Dillustrates a cross-sectional view of the improved pixel cell of thepresent invention through A-A of FIG. 7, where the first metal layer isemployed as a light shield.

[0065]FIG. 8A illustrates a cross-sectional of the improved pixel cellof the present invention after fabrication of active devices byconventional processing methods. The photodiode is formed by the N-welland P- region, and the trench oxide provides a transparent opening toreceive light.

[0066]FIG. 8B illustrates a cross-sectional of the improved pixel cellof the present invention after a first dielectric layer (herein referredto as an inter-layer dielectric (ILD0)) is deposited on the substrateand via lithography and etch has been performed. Via lithographyinvolves coating the dielectric layer with a photoresist material,exposing the photoresist material through a mask, and removing theexposed photoresist in a developing step. The via etch involvestransferring the mask pattern from the photoresist material to the firstdielectric layer. The etching step also involves the removal of thephotoresist material. Via lithography and etch are well known in the artand employ conventional processing steps and processing equipment.

[0067]FIG. 8C illustrates a cross-sectional view of an improved pixelcell of the present invention after chemical vapor deposition (CVD) of ametal (e.g., tungsten) in the vias and a metal polish.

[0068]FIG. 8D illustrates a cross-sectional view of the improved pixelcell of the present invention where the first metal layer (M1) is usedas a metal shield to modify the color responsivity of the pixel. As isseen in FIG. 8D, the metal one layer is configured to restrict theamount of incident light on the predetermined light receiving area.

[0069] FIGS. 9A-9L illustrate a method of manufacturing the improvedpixel cell of the present invention where a metal four layer is employedas a light shield. FIGS. 9A-9L are sectional views through A-A of FIG. 6and FIG. 7. FIGS. 9A-9C are identical to FIGS. 8A-8C and the structuresshown therein are made with the same processing steps.

[0070]FIG. 9D illustrates a cross-sectional view of the improved pixelcell of the present invention after a metal one layer has beendeposited, and the metal one layer lithography and etch processing stepshave been performed.

[0071]FIG. 9E illustrates a cross-sectional view of the improved pixelcell of the present invention after 1) a second dieletric layer (IDL1)has been deposited and polished; 2) a via one lithography and etchprocessing steps have been performed; and 3) a CVD metal deposition andpolish have been performed.

[0072]FIG. 9F illustrates a cross-sectional view of the improved pixelcell of the present invention after a metal two layer has beendeposited, and metal two lithography and etch have been accomplished.

[0073]FIG. 9G illustrates a cross-sectional view of the improved pixelcell of the present invention after 1) a third dielectric layer (IDL2)has been deposited and polished; 2) via two lithography and etch stepsare also performed (although not shown in this cross-sectional view);and 3) a CVD deposition and polish of a metal, such as tungsten, is alsoperformed (although not shown in this cross-sectional view).

[0074]FIG. 9H illustrates a cross-sectional view of the improved pixelcell of the present invention after a metal three layer has beendeposited, and the metal three lithography and etch processing stepshave been performed.

[0075]FIG. 9I illustrates a cross-sectional view of the improved pixelcell of the present invention after 1) a fourth dielectric layer (IDL3)has been deposited and polished; 2) a via three lithography and etchprocessing steps are also performed (although not shown in thiscross-sectional view); and 3) a CVD deposition of metal andcorresponding polish is also performed (although not shown in thiscross-sectional view).

[0076]FIG. 9J illustrates a cross-sectional view of the improved pixelcell of the present invention where the fourth metal layer (M4) isemployed as a light shield to modify the color responsivity of thepixel.

[0077]FIG. 9K is a cross-sectional view of the improved pixel cell ofthe present invention after a dielectric, such as silicon nitride, isdeposited on the fourth metal layer. The silicon nitride is used aspassivation to prevent sodium and moisture from attacking the die.

[0078]FIG. 9L illustrates a cross-sectional view of the improved pixelcell of the present invention after color filter array material has beenspun onto the silicon nitride layer, and the CFA lithography processingsteps of exposure, and development and CFA bake have been performed.

[0079] It will be understood by those skilled in the art that the CFAspin code, lithography and bake process is repeated as many times asthere are different colors in the color filter array (CFA). For example,if the color filter array employs a red color, green color, and bluecolor, the CFA processing steps are repeated three times.

[0080] FIGS. 10A-10C illustrate a cross-sectional view through B-B ofthe improved pixel cell of the present invention where the first metallayer is employed as a light shield.

[0081] FIGS. 11A-11K illustrate the cross-sectional views through B-B ofthe improved pixel cell employing the metal four layer as a lightshield.

[0082] Since the processing steps illustrated in FIGS. 10A-10C and FIGS.11A-K correspond generally to the process flow described with referenceto FIGS. 8A-8D and 9A-9L, the description of the processing steps arenot repeated herein.

[0083] The teachings of the present invention modify the designprinciples for conventional pixel layout. Whereas in conventional pixellayouts, the metal layers are used exclusively for electricalconnection, the present invention teaches that one of the metal layerscan be used for both interconnect and as an optical element to modulatethe color responsivity of pixels. Moreover, whereas conventional layoutrules specifically avoid intrusion into the region above the photodiode,the present invention teaches deliberately routing one level of metal toselectively intrude into the region above the photodiode to reduceincident light and modulate the color responsivity of the pixel.

[0084] As noted earlier, the metal layers and interconnect above andbelow the light modulating layer (or the light shield) can intrude intothe photodiode region to the extent that the light modulating layercovers the predetermined light receiving area. It is important that theother metal layers do not intrude into the non-covered optical pathspecified by the light shield layer. It is also important to note thatthe use of a metal layer for light modulation and as a light shield doesnot preclude that metal layer for use as an interconnect.

[0085] In the preferred embodiment, the pixel is manufactured by anIntel proprietary P854 advanced logic process. Salient features of theP854 process are that it employs multiple metal layers (specifically,four metal layers) and that it exhibits a 0.35 micron (μm) minimumfeature size. For further information regarding the P854 process, pleaserefer to the article entitled “A High Performance 0.35 μm LogicTechnology for 3.3V and 2.5V Operation”, by M. Bohr, S. U. Ahmed, L.Brigham, R. Chau, R. Gasser, R. Green, W. Hargrove, E. Lee, R. Natter,S. Thompson, K. Weldon, and S. Yang, IEDM 94-273-276, Technical Digest.International Electron Devices Meeting, Authors: International ElectronDevices Meeting, IEEE Group on Electron Devices, IEEE Electrical DevicesSociety; Publisher: New York Institute of Electrical and ElectronicsEngineers, c1966; Publ. Year: 1994; Catalog Number: 18752. It will beunderstood by those of ordinary skill in the art that othermanufacturing processes can be employed to make the light shield of thepresent invention.

[0086] In the preferred embodiment, the light shield is a metal layerbecause the metal process is superior to the process for other layerswhen it comes to small features and control of the actual etchingprocess. For example, the P854 process advanced logic process can beemployed to control metal line widths of approximately 0.4 to 1.5microns, depending on the layer number.

[0087] The present invention employs a metal layer as a light shield ora light window. Specifically, the present invention employs the metallayer as a responsivity modulation device. Since the responsivity of acolor pixel is proportional to the light energy (e.g., color of theincident light) and also the intensity of the light incident on anexposed area (e.g., photodiode), the present invention employs metal toaffect the exposed area (i.e., the area than can receive the photons oflight) to compensate for different incident light energy (differentcolors of light) so as to achieve a balanced response to the lightenergy distribution expected for a typical scene.

[0088]FIG. 12 illustrates a perspective view of an image capture system1200 in which the improved sensor of the present invention can beimplemented. The image capture system 1200 includes an IR blockingfilter 1210, a lens assembly 1214 and the imager and package 1218. Theimager includes a sensor circuit 1224. The sensor circuit includes apixel array 1228 having a plurality of pixel cells arranged in rows andcolumns.

[0089] The pixel array, configured with the teachings of the presentinvention, includes a light shield metal layer 1230 having a pluralityof openings where the area of the openings is specifically configuredbased on the color responsivity of the pixel cell. The pixel array alsoincludes a color filter array 1232 having a plurality of elements. Inone embodiment, the color filter array employs three color filtermaterials (red, green and blue).

[0090] In one embodiment, the metal layer includes a plurality ofresponsivity altering windows, where the windows disposed above the bluepixels are approximately equal to 5 microns by 5 microns, the windowsdisposed above red and green pixels are approximately 2 microns by 2micron. The openings employed for controlling the pixel responsivity canrange in size from a minimum set by the fabrication process capabilityto a maximum which results in no blockage of light entering thephotodiode (as shown in FIG. 2). It is anticipated that a typicalembodiment would require openings ranging from approximately one micronby one micron to 5 microns by 5 microns.

[0091] A photosensitive device includes a first area for receivingincident light. The photosensitive device has a responsivity withrespect to the wavelength of the incident light. In other words, theresponsivity of the photosensitive device is dependent on the wavelengthof the incident light. The present invention employs metal toselectively affect the responsivity of the photosensitive device bycovering a portion of the area of photodiode. The exposed area of thered pixels, the green pixels and the blue pixels are selectivelyadjusted so that all of the pixels, regardless of color, saturate atapproximately the same time in response to the light energy distributionexpected for a typical scene. In order to further increase and enhancethe signal to noise ratio of the least sensitive color pixels (i.e., theblue pixels), the present invention allows all the color pixels to beexposed to the light (i.e., control the exposure or integration time) sothat all the color pixels saturate at about the same time.

[0092] The graph corresponding to step 500 in FIG. 5 is a responsivityversus wavelength curve for an unfiltered pixel (i.e., that is a pixelwithout any color filter array material disposed thereon). Theresponsivity is related to the quantum efficiency of the photodiode. Thequantum efficiency (sometimes referred to as effective quantumefficiency) is often expressed as a ratio of the number ofphotoelectrons created by the photoelectric effect to the number ofphotons incident on the entire pixel area even though the active area(the predetermined light receiving area) is typically less than theentire pixel area. The wavelength refers to the wavelength of theincident light.

[0093] The graph corresponding to step 502 of FIG. 5 illustrates atransmittance versus wavelength curve for different color filtermaterials. One can derive the transmittance versus wavelength curves ofa color pixel by convolving the appropriate transmittance versuswavelength curve for that color with a selected responsivity versuswavelength curve. In other words, by convolving the color filter arrayfunction (transmissivity or transmittance versus wavelength) and the IRBlock Filter chromatics, the sensor responsivity versus wavelength curve(quantum efficiency versus wavelength), a net responsivity versuswavelength curve (see graph associated with step 506) that models thebehavior of a color pixel is determined. A fill factor percentage of 31%represents an exposure of the entire active area.

[0094] The horizontal axis is the transmittance, which is the fractionof the incident light that reaches the active area. The transmittancemay be expressed as a percentage, or as a fraction between 0 and 1. Theterm, “transmissivity”, may be loosely applied by those skilled in theart, although a strict definition of transmissivity is the fraction oflight passing through a material per unit thickness. Thus,transmissivity is an intrinsic material property, while transmittance isan extrinsic material property.

[0095]FIG. 13 is a relative response versus wavelength graphillustrating five curves with each curve corresponding to a fill factor(FF) percentage. This graph illustrates how the present inventionreduces the incident light on the photodiode. The fill factor is thepercentage of the total pixel area which is open to incident light. Apixel can be divided into an active area (i.e., the area having thecircuit element to convert light photons into electrons) and areas whereother circuits are located. The fill factor percentage refers to theamount of active area that is uncovered divided by the total pixel area.For example, in this example, an uncovered pixel (i.e., 100% of thetotal pixel area is exposed to light), the fill factor percentage isapproximately 31%. As can be seen from FIG. 13, when the fill factorpercentages are approximately 6.2%, only a small fraction of the lightis detected. As a general rule, as the fill factor percentage decreases,the amount of light detected by the active area correspondinglydecreases.

[0096] There are two principal sources of noise: First, there is shotnoise that varies with the total number of photons collected. A secondsource of noise is related to dark current. Both types of noiseintroduce a statistical uncertainty into the signal captured by thesensor. By increasing the number of photons collected, the shot noise,expressed as a fraction of the captured signal, is reduced. Hence, bylengthening the integration time, shot noise can be reduced.

[0097] The second source of noise is dark current. Dark currentrepresents a leakage current that causes the accumulation of storedelectrons over the integration period. The electrons contributed fromthe dark current cannot be distinguished from photoelectrons, and henceare undesirable. However, in a well-designed sensor, the dark currentcontribution to the signal is smaller than the shot noise. Thus,increasing the number of photoelectrons captured in the least sensitivepixels (by increasing the integration time) benefits the signal to noiseratio. The present invention allows a longer exposure for the benefit ofthe least sensitive pixels without saturation of the more sensitivecolor pixels thereby decreasing the shot noise in the image capturesystem.

[0098] The exemplary embodiments described herein are provided merely toillustrate the principles of the invention and should not be construedas limiting the scope of the invention. Rather, the principles of theinvention may be applied to a wide range of systems to achieve theadvantages described herein and to achieve other advantages or tosatisfy other objectives as well.

1. A pixel comprising: a) a substrate having a photodiode, saidphotodiode having a light receiving area; b) a color filter array (CFA)material of a first color disposed above said substrate, said pixelhaving a first relative responsivity; and c) a light shield disposedabove the substrate, said light shield forming an aperture, saidaperture having an area substantially equal to the light receiving areaadjusted by a reduction factor, said reduction factor being a result ofan arithmetic operation between the first relative responsivity and asecond relative responsivity associated with a second pixel of a secondcolor.
 2. The pixel of claim 1 wherein the reduction factor is theresult of the first relative responsivity divided by the second relativeresponsivity.
 3. The pixel of claim 1 wherein the light shield includesa metal layer.
 4. The pixel of claim 1 wherein the light shield includesan opaque material.
 5. The pixel of claim 4 wherein the opaque materialis a dielectric material.
 6. The pixel of claim 5 wherein the dielectricmaterial includes a silicon dioxide.
 7. The pixel of claim 1 wherein thepixel is a green pixel and the second pixel is a blue pixel.
 8. Thepixel of claim 1 wherein the pixel is a red pixel and the second pixelis a blue pixel.
 9. A method comprising the steps of: a) determining arelative responsivity (S₁) for a pixel of a first color; b) determininga relative responsivity (S₂) for a pixel of a second color; c)determining whether the relative responsivity (S₁) for the first pixelis more than the relative responsivity (S₂) of the second pixel; if yes,forming a mask opening above the first pixel, said mask opening havingan area substantially equal to the light receiving area adjusted by areduction factor, said reduction factor being a result of an arithmeticoperation between the relative responsivity of the first pixel and therelative responsivity of the second pixel; and forming a mask openingabove the second pixel, said mask opening having an area substantiallyequal to the light receiving area; else, forming a mask opening abovethe first pixel, said mask opening having an area substantially equal tothe light receiving area; and forming a mask opening above the secondpixel, said mask opening having an area substantially equal to the lightreceiving area adjusted by a reduction factor, said reduction factorbeing a result of an arithmetic operation between the relativeresponsivity for a second pixel and the relative responsivity of thefirst pixel.
 10. The method of claim 9 wherein the light receiving areais multiplied by the reduction factor.
 11. The method of claim 9 whereinthe arithmetic operation is a division operation.
 12. A method topattern an array comprising the steps of: a) determining a relativeresponsivity (S₁) for pixels of a first color; b) determining a relativeresponsivity (S₂) for pixels of a second color; c) determining arelative responsivity (S₃) for pixels of a third color; d) determiningwhether the relative responsivity (S₁) for pixels of the first color islower than the relative responsivity (S₂) of pixels of the second colorand the relative responsivity (S₃) of pixels of a third color; e) ifyes, forming a mask opening above the pixels of the first color, saidmask opening having an area substantially equal to the predeterminedlight receiving area; forming a mask opening above the pixels of thesecond color, said mask opening having an area substantially equal tothe predetermined light receiving area adjusted by a reduction factor,said reduction factor being a result of an arithmetic operation betweenS₁ and S₂; and forming a mask opening above the pixels of a third color,said mask opening having an area substantially equal to thepredetermined light receiving area adjusted by a reduction factor, saidreduction factor being a result of an arithmetic operation between S₁and S₃.
 13. The method of claim 12 wherein the mask opening formed abovethe pixels of the second color has an area substantially equal to thepredetermined light receiving area multiplied by (S₁/S₂); and the maskopening formed above the pixels of a third color has an areasubstantially equal to the predetermined light receiving area multipliedby (S₁/S₃).
 14. The method of claim 12 further comprising the steps of:a) determining whether the relative responsivity (S₂) for pixels of thesecond color is less than the relative responsivity (S₁) of pixels of afirst color and the relative responsivity (S₃) of pixels of a thirdcolor; b) if yes, forming a mask opening above the pixels of the secondcolor, said mask opening having an area substantially equal to thepredetermined light receiving area; forming a mask opening above thepixels of the first color, said mask opening having an areasubstantially equal to the predetermined light receiving area adjustedby a reduction factor, said reduction factor being a result of anarithmetic operation between S₂ and S₁; and forming a mask opening abovethe pixels of a third color, said mask opening having an areasubstantially equal to the predetermined light receiving area adjustedby a reduction factor, said reduction factor being a result of anarithmetic operation between S₂ and S₃.
 15. The method of claim 12wherein the mask opening formed above the pixels of the second color hasan area substantially equal to the predetermined light receiving areamultiplied by (S₂/S₁); and the mask opening formed above the pixels of athird color has an area substantially equal to the predetermined lightreceiving area multiplied by (S₂/S₃).
 16. The method of claim 12 furthercomprising the steps of: a) determining whether the relativeresponsivity (S₃) for pixels of a third color less than the relativeresponsivity (S₁) for pixels of a first color and the relativeresponsivity (S₂) for pixels of a second color; b) if yes, forming amask opening above the pixels of a third color, said mask opening havingan area substantially equal to the predetermined light receiving area;forming a mask opening above the pixels of a first color, said maskopening having an area substantially equal to the predetermined lightreceiving area adjusted by a reduction factor, said reduction factorbeing a result of an arithmetic operation between S₃ and S₁; and forminga mask opening above the pixels of a second color, said mask openinghaving an area substantially equal to the predetermined light receivingarea adjusted by a reduction factor, said reduction factor being aresult of an arithmetic operation between S₃ and S₂.
 17. The method ofclaim 12 wherein the mask opening formed above the pixels of the secondcolor has an area substantially equal to the predetermined lightreceiving area multiplied by (S₃/S₁); and the mask opening formed abovethe pixels of a third color has an area substantially equal to thepredetermined light receiving area multiplied by (S₃/S₂).
 18. The methodof claim 12 wherein the step of determining the relative responsivity(S₁) for pixels of a first color includes the steps of: a) determiningan input photodiode responsivity; b) determining an input color filterarray transmittance for the first color; c) determining an input IRblocking filter characteristic; d) computing a net response bymultiplying the input photodiode responsivity, the input color filterarray transmittance for the first color, and the input IR blockingfilter characteristics; e) determining an input light source spectralcharacteristic; and f) convolving the net response and the light sourcespectral characteristics to generate the relative responsivity (S₁) forthe first color.
 19. The method of claim 12 wherein the step ofdetermining the relative responsivity (S₂) for pixels of a second colorincludes the steps of: a) determining an input photodiode responsivity;b) determining an input color filter array transmittance for the secondcolor; c) determining an input IR blocking filter characteristic; d)computing a net response by multiplying the input photodioderesponsivity, the input color filter array transmittance for the secondcolor, and the input IR blocking filter characteristics; e) determiningan input light source spectral characteristic; and f) convolving the netresponse and the light source spectral characteristics to generate arelative responsivity (S₂) for the second color.
 20. The method of claim12 wherein the step of determining the relative responsivity (S₃) forpixels of a third color includes the steps of: a) determining an inputphotodiode responsivity; b) determining an input color filter arraytransmittance for the third color; c) determining an input IR blockingfilter characteristic; d) computing a net response by multiplying theinput photodiode responsivity, the input color filter arraytransmittance for the third color, and the input IR blocking filtercharacteristics; e) determining an input light source spectralcharacteristic; and f) convolving the net response and the light sourcespectral characteristics to generate a relative responsivity (S₃) forthe third color.
 21. The method of claim 12 wherein the first color isred, the second color is green and the third color is blue.
 22. A methodfor manufacturing an improved pixel cell that employs a first metallayer as a light shield comprising the steps of: a) forming a substratehaving active devices, said active devices including a photodiode; b)depositing a dielectric layer on the substrate; c) performing vialithography and etch on the dielectric layer; d) depositing a metal inthe via; e) polishing the metal; f) depositing a metal layer on thedielectric layer; and g) performing lithography and etch on the metallayer by employing a metal mask, said metal mask having a plurality ofopenings; wherein the mask opening above pixels of a first color havinga lowest responsivity is equal to the area of the predetermined lightreceiving area; wherein the mask opening above pixels of a second colorhaving a responsivity greater than the responsivity of pixels of thefirst color is equal to the predetermined light receiving areamultiplied by S₁ divided by S₂ where S₁ is the relative responsivity ofthe first color and S₂ is the relative responsivity of the second color;and wherein the mask openings above the pixels of a third color having aresponsivity greater than the responsivity of pixels of the second coloris equal to the predetermined light receiving area multiplied by S₁divided by S₃ where S₃ is the relative responsivity of the third color.